Several agents of microbial nature with the ability to induce plant diseases cause considerable damages, and accordingly economic losses, in crop plants. Many of them attack leaves and/or other aerial plant parts and then usually reach new uninfected crops by airborne spores. Others are transmitted from one crop generation to the next by being seedborne and several economically important disease-inducing agents are soilborne and reside more or less inactive in the soil until a susceptible crop is grown.
Procedures exerted for controlling microbial disease-inducing agents in crop production are often costly, but in most crop growing systems economically necessary. One widely used method is treatment with biostatic or biocidal chemicals. They are in most cases applied as sprays on growing crops, as seed or root treatments or as soil disinfectants. Other standard methods are breeding for resistance and management of the cropping system itself.
These standard control methods all have some drawbacks. Managing of the cropping system is effective or convenient only for certain disease problems. Also the breeding of crop plants for resistance is possible or suitable only in certain cases, may take long time and the resistance obtained may be broken after some time by the appearance of new strains of the pathogen. Chemical compounds often are highly effective, but they may give unwanted effects in the environment, require careful handling as most are risky for human health and they also may become ineffective where resistant pathogen strains develop.
The use of biological control agents or biopesticides may be more effective or more preferable than the use of other control methods and, thus, such agents have been extensively tried. Several bacterial and fungal strains are known to inhibit growth of various microbial disease-inducing agents. To be effective and usable they have to be stable, give reproducible results in the field and there must be possibilities to apply them under field conditions. To date few have fulfilled these requirements and, thus, have been used as commercial products.